In the present specification, reference is made to the following publications illustrating conventional techniques.    [1] WO 2005/098895 A2;    [2] A. Yurtsever et al. “Subparticle Ultrafast Spectrum Imaging in 4D Electron Microscopy” in “Science” vol. 335, 2012, pp. 59-64;    [3] G. Sciaini et al. “Femtosecond electron diffraction: heralding the era of atomically resolved dynamics” in “Reports on Progress in Physics” vol. 74, 2011, pp. 096101-096137;    [4] P. Baum, “On the physics of ultrashort single-electron pulses for time-resolved microscopy and diffraction” in “Chemical Physics” vol. 423, 2013, pp. 55-61;    [5] A. Gahlmann et al. “Ultrashort electron pulses for diffraction, crystallography and microscopy: theoretical and experimental resolutions” in “Physical Chemistry Chemical Physics” vol. 10, 2008, pp. 2894-2909;    [6] M. Aidelsburger et al. “Single-electron pulses for ultrafast diffraction” in “Proceedings of the National Academy of Sciences of the United States of America” vol. 107, 2010, pp. 19714-19719;    [7] A. Gliserin et al. “Compression of single-electron pulses with a microwave cavity” in “New Journal of Physics” vol. 14, 2012, p. 073055;    [8] J. Urata et al. “Superradiant Smith-Purcell emission” in “Physical Review Letters” vol. 80, 1998, pp. 516-519;    [9] G. Doucas et al. “1st Observation of Smith-Purcell Radiation from Relativistic Electrons” in “Physical Review Letters” vol. 69, 1992, pp. 1761-1764;    [10] A. Gover et al. “A Unified Theory of Magnetic Bremsstrahlung, Electrostatic Bremsstrahlung, Compton-Raman Scattering, and Cerenkov-Smith-Purcell Free-Electron Lasers” in “IEEE Journal of Quantum Electronics” vol. 17, 1981, pp. 1196-1215;    [11] G. Adamo et al. “Tuneable electron-beam-driven nanoscale light source” in “Journal of Optics” vol. 12, 2010, pp. 024012-024017;    [12] G. Adamo et al. “Electron-Beam-Driven Collective-Mode Metamaterial Light Source” in “Physical Review Letters, vol. 109, 2012, pp. 0217401-0217406;    [13] I. A. Walmsley et al. “Characterization of ultrashort electromagnetic pulses” in “Advances in Optics and Photonics, vol. 1, 2009, pp. 308-437;    [14] G. Adamo et al. “Light Well: A Tunable Free-Electron Light Source on a Chip” in “Physical Review Letters” vol. 103, 2009, pp. 113901-113905;    [15] N. Talebi et al. “Numerical simulations of interference effects in photon-assisted electron energy-loss spectroscopy” in “New Journal of Physics” vol. 15, 2013, pp. 053013-053028; and    [16] European Patent Application No. 13001598 (not published on the priority date of the present specification).
Electron microscopes have provided so far an efficient tool for investigating the static response of samples at high spatial resolution within the sub-nanometer scale. A transmission electron microscope (TEM) can be operated e. g. in imaging, spectroscopy and diffraction modes. Cathodoluminescence (CL) light detection is also possible in TEM. Scanning electron microscopy (SEM) is based on the collection of secondary and/or back-scattered electrons to form an image or CL to perform spectroscopy. In transmission electron microscopy, the inelastic scattering of the electrons with matter, and the in-situ measurement of the electron energy loss is a powerful technique for mapping the optical density of states; this technique is called low-loss electron energy-loss spectroscopy (EELS).
In addition to static imaging of samples to be investigated and optical density of states, time-resolved spectroscopy of ultrafast processes such as chemical bonding dynamics, macromolecular conforming changes, nanomechanical vibrations, biological sample evolution, and condensed matter systems has become possible by means of conjugate electron-photon sources, with high spatial and temporal resolution [1]-[3]. In these systems, a femtosecond laser source is utilized to both excite the sample with photon pump pulses and to drive an photoemission electron source. The photoemission electrons are then focused onto the sample using static lenses and apertures, to minimize the temporal dispersion due to the space-charge effect and electron-pulse dispersion in vacuum. Setting a series of delays between the incoming electron probe pulse and photon pump pulse by means of an optical delay line, the electrons probe the dynamics of the structural processes with respect to the time reference set by the laser excitation. The time-resolution in electron microscopes is then limited by the electron pulse duration, which is intrinsically controlled by several factors, as described below. Furthermore, the conventional techniques have disadvantages in terms of controllability and structural complexity of the optical delay line for adjusting the delay between the electron probe pulse and photon pump pulse.
Mapping the structural dynamics in ultrafast electron microscopy, diffraction and spectroscopy is achieved by accumulation of several electrons detected at the detector. In order to study irreversible processes, a single pulse containing at least 107 electrons is required to acquire a spectrum or an image with tolerable signal-to-noise ratio, which is referred to as a single shot operating mode in ultrafast electron diffraction and femtosecond electron diffraction [3] methodologies. Incorporating such a dense electron pulse, temporal resolution of the electron pulses at the instance of arrival at the sample is controlled at best within the picosecond regime, due to space-charge effects [3].
Avoiding the space-charge effect by operating the electron microscopes in single-electron-pulse mode is considered as an efficient way to increase the temporal resolution to 150 femtosecond, at best [4, 5]. In this mode accumulation of 107-109 single-electron pulses is required at the detector, depending on the thickness of the sample. Each individual electron then forms an individual point on the screen, similar to Young's double-slit interferometer. In such a concept, the effective time-resolution is dictated by the temporal broadening of the laser beam impinging on the photoemission cathode (τlaser) the geometry of the cathode, temporal broadening due to the applied acceleration voltage (τacc), (temporal dispersion in the vacuum (τdis), and time-jitter (τjitter). The latter term is due to the stochastic arrival time of the single-electron pulses on the sample in comparison with the laser clock. This term is not present in the single-shot operational mode. While the quantum behaviour of individual electron pulses, describable by the Schrödinger equation, is responsible for the longitudinal coherence length of the electron pulses, the effective pulse duration is determined by both the quantum nature and the stochastic nature of the electron pulses. The full temporal resolution of the electron pulses are then described by τ2=τQ2+τjitter2, in which τQ2=τlaser2+τdisp2+τacc2, assuming homogeneous Gaussian broadening for the photoemission process and free-space dispersion [6].
Most optimized electron sources in practical ultrafast electron microscopes offer a temporal resolution of hundreds of femtoseconds, while the temporal coherence is only of the orders of few femtoseconds (about 8 fs) [4]. One can conclude that only a small degree of temporal coherence is present in the series of single-electron pulses as a stochastic assembly, mainly due to the presence of the time-jitter (about 6%). In such a case each electron can only temporally interfere with its own field.
Although the previously mentioned temporal resolution is sufficient for studying many physical dynamics such as nuclear motions in chemical reactions, investigation of electron motions and recombination dynamics demand sub-femtosecond temporal resolution. There have been several proposals to reach the mentioned temporal resolution provoking the concept of a temporal lens. Static solutions for electron pulse compression cannot go further beyond the initial resolution of the electron pulses leaving the photoemission cathode. The few-femtosecond regime is shown to be addressable with electromagnetic compression techniques, either in the form of optical gratings or microwave cavity. However, still the time reference set by the arrival time of the laser in comparison with the electron arrival time is limited by the choice of the synchronization technique between the compressive electron-optical element and the laser source, and even a clear statement on the possible final limit on the temporal resolution due to the time jitter is not present in the literature, mainly because of the lack of theoretical models [7].
Although the single-electron mode in comparison with the single-shot mode has offered a better temporal resolution due to the omission of the space-charge effect, still the single electron mode suffers from the stochastic behaviour of the assembly of at least 107 individual electron pulses needed to carry out the experiment, in comparison with the time reference set by the laser pump illuminating the sample. There is an interest in providing an improved time-reference avoiding the influence of this stochastic behaviour.